When starting a motor it is sometimes desirable to reduce the current supplied to the motor in order to conserve power. This is, e.g., the case when the applied power source is a limited power source, such as a battery or a solar panel. However, there may also be other situations in which it is desirable to conserve power.
However, when the current supplied to the motor is reduced, the rotational speed of the motor decreases. In some motors, such as motors used for driving a compressor, lubrication of the moving parts is ensured by sucking oil and flinging it out by means of the compressor, thereby supplying oil to all the moving parts. In the case that the rotational speed of the motor is below a critical limit, the moving parts of the compressor will not be lubricated sufficiently in this manner, and the compressor will therefore eventually break down if the motor continues to run at the low rotational speed. In order to protect the compressor it is therefore important that the rotational speed of the motor is increased to a level above the critical limit relatively quickly during the start-up. This may be ensured simply by supplying current at a sufficiently high level. However, this would be in conflict with the desire to conserve power.